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Old 7th March 2010, 09:12 PM   #1
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Talking Protection of the Irreplaceable

Greetings, all... First Post, please be gentle!

So, I'm a lucky guy. A recent Spring Cleaning of a client's Transmitter Site netted me a (mostly) working Scott LK 150. It's no slouch in the looks department either, having at been stored in a cabinet away from the dust and grime for however long...I kinda wish their transmitters were that clean.

Most of my previous "projects' have been different levels of rebuilds on fairly common stuff... my Dad's Eico ST70, Scott 350 Tuner, a little Scott 200, and a Fisher 400, among others.

My feeling is that the LK150 is in a different class, only because so few are around, and thus more parts would be made of Unobtanium. So I wonder... is it perhaps wise to do some crazy stuff like...

-fuse the B+?
-fuse the OPT primary sides?

I'm also lucky in that I have many of the parts in house to do a "Safety Fix", like the Bias rectifier and filters, and adding big 10 ohm Cathode resistors to the finals... and probably freshen up the coupling caps as well. The Cans seem to be ok, and B+ is clean and stable.

Unfortunately, a "big giant failure" like a transformer, would put this really nice old amp up for sale as a parts pig, because the cash just isn't there if I pop Iron.

Your thoughts?

(By the way, it is truly astounding to wake one of these beasts on the Variac and listen to it "clear it's throat" after a decades long nap, over several hours. sure... it needs work, and isn't as bright as it should be, but neither am I when I get roused from a long sleep... )

Regards
-art
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Old 7th March 2010, 09:44 PM   #2
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Schematics can be found in this thread
If you are worried about you OPTs, you can fuse them with fast 315mA fuses. Putting 'big' cathode resistors in might not be the smartest move, since they should be acting as fuses in the case of failure. If you want to do the extra cautious thing, you can put 125mA fuses in the cathode legs, too. To protect the power xformer, you might want to double checked it's properly fused at the primary side.
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Old 7th March 2010, 09:49 PM   #3
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Hi Uncle,
Nice find fer sure. Fusing the output transformers at the center tap (red) lead is a good idea. You even have those handy metal stiffening supports to mount fuse holders on either side underneath. Also be sure that the main fuse is of the correct type. I've seen transformers cook because of an oversized "temporary" one that was left in. Fusing the B+ after the rectifier isn't necessary provided the primary fuse is correct. Ten ohm cathode resistors for measuring the tube current need only be one watt carbons. And yes, do replace the coupling caps to the output tubes.
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Old 7th March 2010, 10:23 PM   #4
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Art

Taking GOOD care of the "iron" makes lots of sense. Replace the PSU 'lytics and splitter to "final" coupling capacitors. Test all Carbon composition resistors for drift out of tolerance. Except for grid stoppers, which should remain Carbon comp., replace "shaky" resistors with KOA-Speer Carbon film parts from Mouser. It seems that K-S resistors are the same as Kiwame "audiophile grade", at a much more favorable price.

A look at the schematic shows 7199 small signal tubes. The type is out of production everywhere and going the way of the Dodo is an unfortunate reality. Buy some spares, while you can, or do the small amount of work needed to switch to the more plentiful 6U8.

New Sensor offers "reissue" TungSol 6550s and it seems those are ideal for the LK150. Of course, KT88s will work and New Sensor offers 2 good varieties of that type.
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Last edited by Eli Duttman; 7th March 2010 at 10:25 PM. Reason: punctuation
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Old 14th March 2010, 06:11 PM   #5
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Thank you all for your constructive ideas...

Having literally just gotten inside this thing, my first impressions are that I could do a very minimal amount of work "right now". For a kit, it was well done. A few ugly solder joints, but overall, someone did nice work. (I guess you wouldn't drop almost $200 in 1963 dollars if it was a "first kit")

I will be fusing the OPT secondaries, and yes, sure enough, there was a TWENTY FIVE AMP FUSE in the primary line fuseholder!!! (I spotted this immediately, and replaced it along with what little was left of the power cord. I suppose it could have been worse; i've found 1/4" threaded rod used in some instances...)

New Bias rectifier is in, and the Bias electrolytics are replaced with new "under the deck", but the old can is staying up top to keep the look. I'm gonna need to save up a bit for the other big cans, as I can't bring myself to use plastic-wrapped replacements where they'll be visible.

Tung Sol 6550's are on the way, and yes, I need to lay in a stock of 7199's. I will likely get a set of those silly adapters to use 6U8's and save on 7199s, but on an item this "rare", I do not think I will make any permanent wiring changes.

So far, I am most fortunate; the "cosmetic cleaning" has required nothing besides a mild soap/water wipe with a rag, and the chassis is in very shiny shape.

Thanks again... I hope to get to work and post some pics somewhere soon.

Regards
-art
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Old 14th March 2010, 09:59 PM   #6
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Quote:
New Bias rectifier is in, and the Bias electrolytics are replaced with new "under the deck", but the old can is staying up top to keep the look. I'm gonna need to save up a bit for the other big cans, as I can't bring myself to use plastic-wrapped replacements where they'll be visible.
Since the amp was new, capacitance/unit volume in 'lytics has gone way up. It is possible to replace the innards of the OEM can caps., using current production parts. A pipe/tubing cutter is employed to separate the can from the base. Through the contents out, clean the inside of the can, and stuff good quality (Panasonic, Nichicon, etc.) parts into the space made available. Wire to the base, carefully, and glue (epoxy, RTV, ...) the can back on the base.

Quote:
I need to lay in a stock of 7199's.
Jim McShane charges $36.95 per tube for Philips JAN.
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Old 14th March 2010, 11:47 PM   #7
chrish is offline chrish  Australia
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Reading the title of the thread, I thought you were talking about a part of your anatomy!
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Old 15th March 2010, 01:02 AM   #8
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Originally Posted by chrish View Post
Reading the title of the thread, I thought you were talking about a part of your anatomy!
Haven't priced one of those lately...
Put protection is just as important...
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